1. Field of the Invention
The present invention relates to a terminal apparatus which performs communication via a communication medium. More particularly, the present invention relates to a terminal apparatus which is used in a system which controls an opportunity to access a communication medium.
2. Description of the Background Art
As broad-band access line infrastructures are built and widespread, and home network appliances are developed, there is a demand for construction of an infrastructure for a home network having an access line with high transmission ability.
If an inexpensive and high-speed digital network can be constructed as a home network infrastructure without laying a new physical cable (hereinafter referred to as a physical communication dedicated cable), the user will have a great benefit. As a method which does not require a new physical communication dedicated cable, a method of using an existing wired cable, such as a power line, a telephone line, a television antenna line, or the like, for high-frequency communication, a method of providing a wireless network, and the like have been proposed. These communication media are not dedicated media for high-speed digital communication, so that a sufficient band may not be provided and the reliability of data transmission may be poor.
In such a communication system which uses a communication medium having a problem in communication quality, an efficient and effective network protocol is required. Conventional network protocols handle terminals on a network equally. Therefore, each terminal needs to subjectively access a communication medium. In other words, conventional network protocols are each confined as a process within a single terminal. Therefore, data transmission delay or data jitter is likely to occur.
However, when video, audio, or voice information is transmitted in packets, delay or jitter needs to be strictly limited. In conventional network protocols, packets containing video, audio, or voice information cannot be transmitted with high quality.
In order to achieve an efficient control of traffic of a whole network, a network protocol has been developed, in which a terminal of a logical network is used as a control terminal for controlling medium access of other terminal(s), i.e., the other terminals are controlled terminals (hereinafter referred to as a centralized control type medium access control method).
In the centralized control type medium access control method, the control terminal controls opportunities of access of other terminal(s) to a communication medium. Therefore, the efficiency of use of the medium on the whole network is significantly influenced, depending on which terminal within a single logical network serves as a control terminal. Therefore, it is important and difficult to select a control terminal.
Such a problem is conventionally solved in, for example, the conventional centralized control type medium access control method by causing each terminal of a network to recognize the number of terminals with which that terminal can perform direct wireless communication, and changing control terminals so that a terminal having the largest number of terminals with which that terminal can perform direct wireless communication serves as a control terminal. By dynamically selecting a control terminal in this manner, a traffic control of a whole network can be efficiently performed.
Japanese Patent Laid-Open Publication 2000-138685 discloses a conventional centralized control type medium access control method in which a terminal activated within a network is caused to perform carrier sense for a predetermined period, and if the terminal cannot recognize a frame(s) issued by other terminal(s) in a channel (frequency transmission channel) which is used in communication, during the carrier sense, the activated terminal is caused to operate as a control terminal. By automatically setting a control terminal at an early stage of construction of a logical network in this manner, a smooth operation of the logical network can be achieved.
However, in the conventional centralized control type medium access control method, there is a possibility that a plurality of logical networks are automatically created within a single communication medium which is controlled by a user. Particularly, when the communication medium is a power line, a plurality of logical networks may be created in the communication medium due to a short-term transmission channel failure or the specificity of power line wiring.
When a plurality of logical networks are created in a single communication medium controlled by the user as described above, data transmission between terminals may be redundant against the user's intention. The reason is the following. It is assumed that two logically different logical networks are physically adjacent to each other, and there is a transmission channel of a communication medium via which two terminals belonging to the respective two different logical networks can perform direct communication. In such a case, the centralized control type network control method cannot achieve direct communication between terminals of the different logical networks unless a particular technique is used. Therefore, by causing a control terminal to relay a frame, it is possible to achieve communication between terminals of the different logical networks. When a frame is transmitted via a control terminal in this manner, the use of the communication medium has redundancy. Therefore, when a plurality of logical networks are created, data transmission between terminals may be redundant against the user's intention.
In other words, the centralized control type network control method aims efficient use of a communication medium within a single logical network. Therefore, when a plurality of logical networks are created adjacent to each other, the efficiency of use of the communication medium may be reduced, resulting in impairment of network service which is expected by the user.